1. Field of the Invention
The present invention relates to a tracking antenna system mounted on a mobile for tracking a radio source, and more particularly to a tracking antenna system which tracks a target on the basis of both strength of received radio wave and a turning angular rate of the mobile.
2. Description of the Related Art
A tracking antenna system for tracking a radio source, e.g. a satellite by using both step track control, in which tracking is carried out based on the receiving signal level while changing the beam direction of the antenna by a predetermined amount, and gyro control, in which the turning angular rate of the mobile is used to track the target, has been known. Hereinafter, a control operation based on an output from a gyro which detects a turning angular rate of a mobile will be referred to as "gyro control"; whereas a control operation using the detected receiving signal level while changing the beam direction of the antenna by a predetermined amount will be referred to as "step track control". In such a tracking antenna system, a control operation using both receiving level signal and angular rate signal is performed when the satellite can be seen, whereas a control operation using only the angular rate signal is performed when a radio wave from the satellite is blocked by buildings, tunnels and so on.
However, the tracking antenna system of the above-mentioned type raises, for example, the following problem: if only the gyro control is performed for a certain time period while a radio wave is blocked, the beam direction of the antenna gradually deviates from the satellite direction, due to effects of an error contained in a gyro output, and the antenna eventually loses the satellite. As a result, it takes a long time to search the direction of the satellite when the satellite comes into sight again.
Japanese Laid-Open Patent Publication 5-142321 discloses an approach for overcoming such a problem of an error contained in a gyro output as described above. In this approach, a triangular wave which increases or decreases in synchronization with a binary signal (a control amount for antenna rotation with a positive or negative constant step) generated in the step track control, is superposed on a gyro output. The signal on which a triangular wave is superposed is then used to increase or decrease the rotation angular rate of the antenna. The above approach utilizes the fact that, since the length of the positive period of the binary signal is different from the length of the negative period of the binary signal when a gyro output contains an error, the average value of the triangular wave gradually changes and eventually converges on an error in the gyro output. It is thus possible to correct the gyro output. Further, when the radio wave is blocked, binary signals are generated alternately at a constant cycle to maintain the previous correction state. The gyro output is corrected as described above, so that the tracking of the satellite can be appropriately performed.
In the foregoing prior art, however, the antenna continues to move around the axis running in the satellite direction, even after the average value of the triangular wave coincides with the gyro output error. As a result, if the antenna system is used in a place where the peak value of a receiving level is low, the receiving level lowers when the azimuth of the antenna beam deviates from the satellite direction, which generates noise in a TV screen display.